Method of removing the polymer encapsulating a nuclear fuel pellet

ABSTRACT

Method for removing the epoxy and/or phenolic polymer encapsulating a nuclear fuel pellet comprising uranium dioxide UO 2 , the method comprising the following successive steps: a) the polymer is pyrolysed in a reducing atmosphere; and b) the carbon residues obtained after the pyrolysis step (a) are selectively oxidized, the oxidation being carried out at temperature above 1000° C. in an atmosphere comprising carbon dioxide CO 2 . Such a method makes it possible to remove the epoxy and/or phenolic polymer encapsulating the pellet while avoiding or limiting the risk of radiological contamination by the formation of U 3 O 8 .

FIELD OF THE INVENTION

This invention pertains to the field of nuclear fuel treatment.

In particular, it relates to the treatment, before or after theirirradiation, of nuclear fuel pellets which are encapsulated within apolymer for the purpose of their characterization in metallographicstudies.

BACKGROUND OF THE INVENTION

The microstructure of a nuclear fuel pellet is determined inmetallographic studies using techniques such as optical microscopy orelectron microscopy.

For such studies, it is necessary to prepare the pellet by encapsulatingit with a polymer such as an epoxy and/or phenolic polymer and/orpolyester.

Once the studies have been carried out, the polymer, if some remains onthe pellet, is irradiated by the ionizing radiation emitted by thenuclear fuel. It then degrades through a radiolysis reaction, whoseproducts are in particular explosive gases, such as hydrogen, oxygen andmethane.

For the purposes of shipping, storing and warehousing the pellet, it istherefore necessary to remove the polymer encapsulating all or part ofthe nuclear fuel pellet in order to suppress the risk of explosion,reduce the volume of waste that can be constituted by the encapsulatedpellet, and avoid the polymer's neutron moderator effect in cases wherethe pellet is reused as a nuclear fuel.

Outside the nuclear field, different methods are available for theremoval of a polymer. Thus, in the field of plastic processing, thepolymer covering the matrix used for the injection molding of a part isremoved using a thermal treatment in which the polymer is pyrolyzed inthe presence of an oxidizing gas.

This pyrolysis generates volatile byproducts, which are easily removedthrough post-combustion in an auxiliary oven. However, it also generatessolid residues, the destruction of which requires an additional step ofair oxidation.

A pyrolysis treatment, when applied to a nuclear fuel pellet comprisinguranium dioxide UO₂, should allow as thorough a removal as possible ofthe polymer, but also of the solid residues that are generated bypyrolysis and which remain in contact with the pellet, since radiolyzingsuch residues may also generate explosive gases.

However, a treatment which combines pyrolysis and oxidation steps hasthe drawback of oxidizing UO₂ into U₃O₈. This transformation isaccompanied by a strong increase in the volume of the crystal lattice(by approximately 36), which the sintered pellet cannot accommodate.This leads to significant cracking and swelling of the pellet, therebyresulting in its fragmentation and the generation of U₃O₈ powder.

However, U₃O₈ in powder form has the drawbacks that it can bedisseminated in the air and cause the total release of the fission gasesthat were initially contained in the nuclear fuel pellet (such asKrypton 85), with both phenomena increasing the risks of radiologicalcontamination.

SUMMARY OF THE INVENTION

It is accordingly one of the objects of this invention to provide amethod for removing a polymer (or any residue originating therefrom)encapsulating a nuclear fuel pellet comprising uranium dioxide UO₂, sucha method being capable of avoiding or strongly limiting the generationof O₃O₈ powder as a source of radiological contamination.

The subject matter of the present invention thus relates to a method forremoving the epoxy and/or phenolic polymer encapsulating a nuclear fuelpellet comprising uranium dioxide UO₂.

The method comprises the following successive steps:

a) pyrolyzing the polymer in a reducing atmosphere;

b) performing the selective oxidation of the carbon-based residuesobtained after the pyrolysis step (a), the oxidation being carried outat a temperature above 1000° C. (typically in the range between 1000° C.and 1700° C.) in an atmosphere comprising carbon dioxide CO₂.

Throughout the present description, by “residues” it is meant anyproduct resulting from the epoxy and/or phenolic polymer pyrolysisreaction. In general, these are solid residues, which are said to be“carbon-based residues”, since they contain at least 80% by weight ofcarbon, which may be linked to oxygen or hydrogen atoms.

As for the term “pyrolysis”, it designates a thermal treatment performedin the absence of oxygen.

Preferably, the reducing atmosphere comprises an inert gas (i.e.chemically inert with respect to the chemical species present), such asargon, and hydrogen. The latter preferably represents a proportion lyingbetween 0.5% and 10%, and more preferably approximately 5%, of thevolume of the reducing atmosphere.

Advantageously, the pyrolysis is performed at a temperature above 600°C. (typically in the range between 600° C. and 1700° C.) in order toreduce the amount of residues and residual hydrogen.

The oxidation step (b) is selective in that it is capable of oxidizingthe carbon-based residues resulting from the pyrolysis step (a), whileavoiding or strongly limiting the oxidation of UO₂ into U₃O₈. This stepcould also, when appropriate, be performed under an atmosphere whichfurther comprises carbon monoxide CO, this gas having the advantage ofpreventing the generation of U₃O₈.

Generally, the nuclear fuel pellet treated by the method according tothis invention is constituted mainly of uranium dioxide UO₂. However,when metallographic studies are performed on a nuclear fuel pellet ofthe MOX type (“Mixed OXide fuel”), the method of this invention may alsobe carried out for removing the polymer encapsulating a nuclear fuelpellet that further comprises plutonium dioxide PuO₂.

Other objects, features and advantages of the present invention willbecome more apparent from the following description, which is given byway of non-limiting example with reference to the appended FIGS. 1 and2.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the variation in mass as a function of time ofa UO₂ pellet encapsulated with an epoxy polymer, when the pellet issubjected to pyrolysis under argon U, followed by oxidation in air.

FIG. 2 is a graph showing, for different temperatures, the change inmass as a function of time for the same pellet pyrolyzed in argon U,when it is subjected to oxidation under CO₂.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The following detailed description relates to the removal of an epoxypolymer encapsulating a pellet of uranium dioxide UO₂.

Experiments performed in parallel (not shown here) provide similarresults when the polymer is a phenolic polymer.

1—Manufacture of Encapsulated Pellet Samples.

Uranium dioxide UO₂ obtained by a “wet process” is compressed and thensintered under a reducing atmosphere so as to obtain a pellet with adensity of 10.96 g/cm³.

The pellet is then placed in a mold, and thereafter encapsulated bycasting an epoxy polymer (DGEBA polymer—bisphenol A diglycidylether—marketed by STRUERS as EPOFIX).

After hardening of the polymer, the encapsulated pellet is cut by meansof a wire saw and one of its faces is finely polished (mirror finish) toallow possible microscope observation.

The pellet is 8 mm in diameter and 3 mm in thickness. It is made of 1.05g of UO₂ and 0.23 g of epoxy polymer.

Samples of the encapsulated pellet are placed in a thermobalance (modelSTA 409 C available from NETZSCH), which allows the change in mass of asample subjected to a predefined temperature under a controlledatmosphere to be monitored continuously.

2—Treatment of Encapsulated Pellet Samples

Various methods are implemented to remove the epoxy polymerencapsulating the samples manufactured as above.

2.1—Implementation of the Method of the Invention

This exemplary embodiment of the method according to this inventionunderlines in a comparative fashion, the advantages provided by thepyrolysis step of the invention with respect to pyrolysis underoxidizing conditions.

As a reference, a first sample is pyrolyzed at 600° C. for 2 hours in aslightly oxidizing atmosphere consisting of industrial grade argon(argon U, with impurity contents, expressed in ppm by volume, listed inTable 1).

TABLE 1 N₂ O₂ CO₂ H₂O H₂ CnHm <20 ppm <5 ppm <5 ppm <5 ppm <10 ppm <5ppm

This sample is then subjected to oxidation at 1000° C. for 1 hour in anatmosphere composed of carbon dioxide.

A second sample is treated according to the method of this invention:pyrolysis at 600° C. for 2 hours in a reducing atmosphere composed ofargon, comprising 5% by volume of hydrogen, followed by oxidation at1000° C. for 1 hour under an atmosphere composed of carbon dioxide(under the same oxidation conditions as for the first sample).

Following these treatments, both samples no longer contain any residuesand maintain their mechanical integrity.

However, only the second sample, having been subjected to pyrolysis in areducing atmosphere according to the method of this invention, does notshow any significant mass gain, which would reveal gradual oxidation ofthe UO₂ into U₃O₈. Such a mass gain (expressed in mass percent withrespect to the pyrolyzed sample's mass) is indeed negligible, since itis 0.1% (as opposed to 0.5% for the first sample pyrolyzed under argonU).

2.2—Oxidation Step

To illustrate the advantages achieved by the oxidation step aloneaccording to the present invention, two separate treatments areperformed. However, these treatments differ from the method according tothe present invention in that the pyrolysis step is not performed in areducing atmosphere.

Two samples are subjected to the same pyrolysis under slightly oxidizingconditions (600° C. for 2 hours under argon U), and then to oxidationperformed under different conditions for each sample, namely:

-   -   the first sample is oxidized at 600° C. for 45 minutes in an        atmosphere composed of reconstituted air (20% oxygen and 80%        nitrogen, by volume)    -   the second sample is oxidized, according to the method of the        present invention, at 1100° C. or 1200° C. or 1350° C., for 1        hour in an atmosphere composed of carbon dioxide.

FIG. 1 shows the mass variation of the first sample with time.

FIG. 2 shows this variation for the second sample, as well as thetemperature applied as a function of time (dashed lines). However, itdoes not illustrate the pyrolysis step performed in argon U because thisstep is identical to that shown in FIG. 1. The mass variation it showsis therefore that of the sample after it has been pyrolyzed in argon U.

When analyzing FIGS. 1 and 2, the following differences betweenoxidation in air and oxidation in CO₂ appear clearly.

As apparent from FIG. 1, pyrolysis in argon U removes a large proportionof the polymer (with a mass loss of the encapsulated pellet of the orderof 17%).

Thereafter, when oxidizing the residues in air, the mass loss continuesat a moderate rate until it reaches 200 minutes of treatment. It isquickly compensated by a mass gain estimated to be 3.2% with respect tothe sample's mass before pyrolysis. This is due to the gradual oxidationof UO₂ into U₃O₈, the latter being an oxide of higher molecular weight.This observation is confirmed by the sample's appearance, which is nowentirely in the form of a powder.

As previously mentioned, it is therefore not possible to oxidize thepyrolysis residues in air without at the same time transforming thesintered UO₂ pellet into O₃O₈ powder.

When the oxidation step is performed in CO₂ (see FIG. 2), for all of thetemperatures under consideration, a mass loss occurs, which is relatedto the oxidation of the residues (after 100 minutes). Experimentsperformed at other temperatures (not shown here) show that thisoxidation becomes complete only at a temperature above 1000° C., thistemperature also having the advantage of enhancing the stability of UO₂with respect to U₃O₈.

This mass loss is followed by a mass gain due to the UO₂ pelletoxidation, which leads to the formation of U₃O₈ powder (after 150minutes).

However, the mass fractions of U₃O₈ powder are then much smaller thanthe aforementioned values since, for temperatures of 1100° C., 1200° C.and 1350° C., they are 1.7%, 2.1% and 1.4%, respectively, with respectto the sample's mass after pyrolysis. Such values are insufficient tojeopardize the mechanical integrity of the sintered pellet, whereas theoxidation treatment in air results in the sample being entirelytransformed into a powder.

When comparing both treatments, it therefore appears that the oxidationstep according to the present invention, namely oxidation under CO₂ at atemperature above 1000° C., leads to complete oxidation of the residueswhile strongly limiting the formation of U₃O₈ powder.

As shown in the above description, the method according to the presentinvention allows the epoxy and/or phenolic polymer encapsulating anuclear fuel pellet comprising uranium dioxide UO₂ to be removed, whileat the same time limiting or even avoiding the risk of radiologicalcontamination through the formation of U₃O₈ powder.

1. A method for removing the epoxy and/or phenolic polymer encapsulatinga nuclear fuel pellet comprising uranium dioxide (UO₂), the methodcomprising the following steps: a) pyrolyzing said polymer in a reducingatmosphere; b) performing the selective oxidation of the carbon-basedresidues obtained after the pyrolysis step (a), said oxidation beingcarried out at a temperature above 1000° C. in an atmosphere comprisingcarbon dioxide (CO₂).
 2. The method according to claim 1, wherein saidreducing atmosphere comprises an inert gas comprised of argon andhydrogen.
 3. The method according to claim 2, wherein the hydrogenrepresents a proportion lying between 0.5% and 10% of the volume of saidreducing atmosphere.
 4. The method according to claim 1, wherein saidpyrolysis is performed at a temperature above 600° C.
 5. The methodaccording to claim 1, wherein said oxidation is performed in theatmosphere which further comprises carbon monoxide (CO).
 6. The methodaccording to claim 1, wherein said pellet further comprises plutoniumdioxide (PO₂).